[RFC PATCH 0/5] RISC-V: Add dynamic TSO support

[Christoph Müllner]

On Mon, Nov 27, 2023 at 11:37 AM Conor Dooley
<conor.dooley at microchip.com> wrote:
>
> Hi,
>
> On Fri, Nov 24, 2023 at 08:21:37AM +0100, Christoph Muellner wrote:
> > From: Christoph Müllner <christoph.muellner at vrull.eu>
> >
> > The upcoming RISC-V Ssdtso specification introduces a bit in the senvcfg
> > CSR to switch the memory consistency model at run-time from RVWMO to TSO
> > (and back). The active consistency model can therefore be switched on a
> > per-hart base and managed by the kernel on a per-process/thread base.
> >
> > This patch implements basic Ssdtso support and adds a prctl API on top
> > so that user-space processes can switch to a stronger memory consistency
> > model (than the kernel was written for) at run-time.
> >
> > I am not sure if other architectures support switching the memory
> > consistency model at run-time, but designing the prctl API in an
> > arch-independent way allows reusing it in the future.
> >
> > The patchset also comes with a short documentation of the prctl API.
> >
> > This series is based on the second draft of the Ssdtso specification
> > which was published recently on an RVI list:
> >   https://lists.riscv.org/g/tech-arch-review/message/183
> > Note, that the Ssdtso specification is in development state
> > (i.e., not frozen or even ratified) which is also the reason
> > why I marked the series as RFC.
> >
> > One aspect that is not covered in this patchset is virtualization.
> > It is planned to add virtualization support in a later version.
> > Hints/suggestions on how to implement this part are very much
> > appreciated.
> >
> > Christoph Müllner (5):
>
> I know this is an RFC, but it could probably do with a bit more compile
> testing, as:
>
> >   RISC-V: Add basic Ssdtso support
>
> This patch doesn't build for rv64 allmodconfig
>
> >   RISC-V: Expose Ssdtso via hwprobe API
>
> This one seems to build fine
>
> >   uapi: prctl: Add new prctl call to set/get the memory consistency
> >     model
> >   RISC-V: Implement prctl call to set/get the memory consistency model
> >   RISC-V: selftests: Add DTSO tests
>
> These don't build for:
> rv32 defconfig
> rv64 allmodconfig
> rv64 nommu

Thanks for reporting this. You are absolutely right.
In my defense, this patchset was compile-tested and got some limited
run-time testing in QEMU.
But after that, I wrote the documentation, which triggered a renaming
of several function/macro names,
and these changes did not see adequate testing. I am sorry for that.

I've already fixed the patches (addressing the issues you have
reported, plus other small issues).
To not distract the ongoing discussion, I will not send an updated
patchset right now.
In case you are interested, you can find the latest changes (rebased
on upstream/master) here:
  https://github.com/cmuellner/linux/tree/ssdtso
I've also extended my local compile-test script to include all
mentioned configs.

In case you want to play a bit with these changes, you can also have a
look at the QEMU
patchset, which also got support for the prctl (which is not part of
the published mailpatch):
  https://github.com/cmuellner/qemu/tree/ssdtso
With these changes, you can run the kernel self-test binary in
user-mode emulation.

BR
Christoph